The name of Casteil comes from Latin Castellum who indicated
a Roman castrum then; i.e. a strengthened station. This tower became
with the Middle Ages a strong castle. The name of the village appears
for the first time at XIIè sciècle in the form of Castellum, then in
1358 pennies the form of Casteil whose topomyme means castle.

The origin of Casteil is from the start related to its inheritance.
The oldest mention goes back to 885 and the existence of a castle concerns,
the "castrum verneto". The origin of the village east thus related to the
extremely primitive castle, located on a rocky outcrop above the current
village on the way of the abbey. This fortress was probably located
meadows of Vault of Saint Martin's day the old man. The site had been
strategically selected in order to to supervise the whole of the valley
of Cady. The village was not whereas a simple farm or "villae". These
"villae" was great agricultural properties offered to the vétérants of
the Roman army in thanks of the years spent to the service of the
Empire. In spite of the fall of the Empire, they had maintained them
agricultural populations and their activities, but also structured
the territory. They were at the origin of many villages as it was the
case for Casteil. The primitive castle was then named in 937 "castrum
Sancti Martini", attesting presence of the vault now called
"Martin Saint the Old man. This vault was useful to the inhabitants
of the castle and testifies to the strong demography to the mountains
to the Middle Ages. Some years after, the mention "domus Sancti
Martini" probably attests beginnings of work abbey.